Methods for crosslinking cellulose are well known. In conventional methods for crosslinking cellulose, cellulose hydroxyl groups are reacted with a crosslinking agent having at least two functional groups that are reactive toward the cellulose hydroxyl groups. Traditional crosslinking agents include dialdehydes, such as glutaraldehyde, which provide acetal crosslinks, and polycarboxylic acid crosslinking agents, such as citric acid, that provide ester crosslinks.
Carboxylated celluloses may be crosslinked either through the cellulose hydroxyl groups, or by using a crosslinking agent that is reactive toward the cellulose carboxylic acid groups. Crosslinking agents useful in crosslinking carboxylated cellulose through its carboxyl groups include crosslinking agents having two or more hydroxyl groups, so as to provide diester crosslinks, and crosslinking agents that include two or more amino groups, so as to provide diamide crosslinks. Although diamide crosslinks are more stable than diester crosslinks, amide formation is oftentimes more difficult than ester formation.
Typically, amides are prepared by coupling an amine with an acid chloride derived from a carboxylic acid. Although acid chlorides are highly reactive, the preparation of an acid chloride from a carboxylic acid in large scale poses significant difficulties due to the reagents necessary for making the acid chloride. Most importantly, because acid chlorides are sensitive to water, and because cellulose modification is often carried out in aqueous medium, acid chlorides are not suitable for the formation of cellulose amides. Amidation methods using acid anhydrides as reactive intermediates are also known. However, like acid chlorides, acid anhydrides are also difficult to prepare in aqueous media.
The disadvantages of the use of acid chlorides and anhydrides in amidation methods has caused the development of alternative synthetic processes for amidation. One approach involves the generation of an activated carboxylic acid intermediate that is then treated with an amine in situ to form an amide product.
Recently, a process for triazine-promoted amidation of carboxylic acids has been developed. In the method, amides are prepared from carboxylic acids using a triazine reagent as a promoter. In the method, 2,4,6-trichloro-1,3,5-triazine (also known as cyanuric chloride) is treated with three equivalents of a carboxylic acid in the presence of base in a polar organic solvent to provide the activated carboxylic acid derivative. To the activated carboxylic acid derivative is added an amine in an amount that is a slight excess relative to the carboxylic acid. The product of the reaction is the corresponding amide that is readily separated from the cyanuric acid by-product.
Despite the advances in the development of amidation processes, a need exists for the formation of cellulose amides in aqueous environments typically used for cellulose modification. The present invention seeks to fulfill this need and provides further related advantages. The present invention provides a method for the amidation of cellulose promoted by triazine reagents. In the method, a cellulose carboxylic acid is converted to a cellulose amide by reaction of the carboxylic acid group with a triazine reagent to provide an activated carboxylic acid derivative in situ that is then reacted with an amine to provide a cellulose amide. In the method of the invention, the modification of the cellulose carboxylic acid is carried out in an aqueous environment.